MAGMA FLOW ABOVE THE KAROO MANTLE PLUME

The magnetic fabric of plutonic rocks has often been used as a proxy for magma flow. Yet, a number of studies suggest that the relationship between the principal axes of AMS and flow axes are complex. Several case studies on mafic dikes show that complications may arise from the contribution of SD magnetite grains, imbricated symmetric fabrics along dike margins and post-solidification thermal stresses. Fewer investigations have been dedicated to subhorizontal tabular intrusions despite the fact that they could also yield valuable clues regarding the various processes that might cause anomalous AMS fabrics. The Karoo Large Igneous Province in South Africa hosts a remarkably impressive set of undeformed, stacked gabbroic sills that were intruded parallel to bedding in the Karoo Basin. The sills range in thickness from 1 to 1000 m and have a relatively constant petrological composition of gabbros and gabbro-norites. Theses sills are distributed throughout the whole Karoo Basin and were emplaced at various stratigraphic heights / depths in the Karoo stratigraphic column. Oriented cores were collected from 30 sills and yielded 1598 specimens for AMS, AARM and paleomagnetic measurements. The low-field magnetic susceptibility K_m ranges widely from 100 to 20,000 x 10-6 [SI], while P' ranges from 1.01 to 1.10. Thermomagnetic experiments reveal that the main magnetic carrier is titanomagnetite with variable ulvöspinel content. This is confirmed by measurement of hysteresis properties that also indicate that titanomagnetite in general has a pseudo-single domain grain size. The directional data is consistent with the nearly horizontal attitude of the sill in 23 out of 30 sills, with subvertical K₃ axes. In 5 out of 30 sills, K₃ axes are subhorizontal, characterized by scattered directional data and are considered anomalous AMS fabrics. K₁ axes are systematically subhorizontal.